This invention relates, in general, to leadframes used in manufacturing semiconductor devices, and more particularly, to a method of manufacturing leadframes to eliminate silver bleed on the leads themselves.
Leadframes are typically made from a flat sheet of leadframe material. The leadframes are either stamped, punched, or etched from the sheet of material. A portion of the leadframes is normally silver plated to be compatible with semiconductor die attach and wire bond systems. The portion of the leadframe that needs to be plated is a flag area and the portion of the leads themselves which is adjacent to the flag area. It is this portion of the lead which is wire bonded. Wire bonding consists of bonding a wire between each individual lead of a semiconductor die which is located on the flag area.
Once the leadframe is formed, the plating operation is performed, and then the flag area of the leadframe is down set and the leadframes are cut into desired groups. The group may consist of any desirable number of individual leadframes desired such as six or eight, or the like. The down set operation lowers the flag area so that once the semiconductor die is bonded to the flag area it will be at the same height or elevation as the leads themselves.
During the silver plating operation a mask is placed on the top and the bottom of the leadframe in an attempt to avoid plating portions of the leadframe which are not to be plated. However, during plating silver will bleed along the edges between the leads underneath the masking clamp. After the encapsulation the silver that bleeds along the edges of the leads can then form silver dendrites or silver migration between the leads themselves when a voltage potential is applied between the leads or when the leads are exposed to a humid environment. This will cause an electrical short between the leads resulting in an electrical failure. In an attempt to remove the silver bleed between the leads the encapsulated leadframes are placed in a descaling solution or etchant to remove the silver bleed. Sometimes this results in undercutting the silver thereby creating silver slivers. Therefore it should be appreciated that it would be desirable to provide a method of manufacturing leadframes which totally eliminates silver bleed.
Accordingly, it is an object of the present invention to provide a method of eliminating leadframe silver bleed.
Another object of the present invention is to provide a method of eliminating leadframe silver bleed which does not involve a chemical process.
Yet another object of the present invention is to provide a cost effective method to eliminate silver bleed between the leads of the leadframe.